The methods known as “jet grouting” are used to form columnar artificial conglomerate structures in the soil. These methods are based on the mixing of particles of the soil itself with binders, usually cement mixtures, which are injected at high pressure through small radial nozzles formed in an injection head (commonly referred to as a “monitor” in this technical field) fixed in the proximity of the lower end of a set of tubular rods which is rotated and withdrawn towards the surface. The jets of binder are dispersed and are mixed with the surrounding soil, thus creating a conglomerate body, generally of cylindrical shape, which, when hardened, forms a consolidated area of soil. The dispersing efficacy of the jet can be increased by the addition of injected water and/or pressurized air.
At the bottom of the set of tubular rods, under the monitor, there is fixed a drilling tool which is lubricated, during the excavation phase, with a drilling fluid supplied through the rod.
In the methods used up to the present time, both the consolidating mixture and the drilling fluid are supplied, in successive phases, through the same pipe inside the rod. When the desired depth is reached, the supply of the drilling fluid is halted and the injection of the consolidating mixture commences. The flow towards the drilling tool is blocked by a ball valve or an automatic valve, thus diverting the whole flow of the consolidating mixture towards the lateral nozzles.
Progress has recently been made in the limiting of the pressure losses which occur inside the monitor. These pressure losses are due to turbulence caused by the abrupt change of direction (from vertical to horizontal) inside the monitor, and reduce the efficiency of the system.
In a recently proposed system, the consolidating mixture is diverted from the vertical pipe to the lateral nozzles through one or more curved pipes which convey the fluid streams along a path having a gradual change of direction, thus reducing turbulence. In U.S. Pat. No. 5,228,809, use is made of a pipe of constant cross section and regular curvature. In EP 1 396 585, use is made of pipes of variable curvature which are progressively tapered.
In these cases also, the drilling process requires a supply of drilling fluid to the tool or bit during the initial drilling phase, while the injection of the consolidating mixture takes place subsequently during raising. Since the known systems for producing high-efficiency jet tools (known as “high-efficiency injection heads”) are characterized by the continuity of the pipe from the rod to the nozzle, it is necessary to provide another channel to be used solely for the flow of the drilling fluid in order to lubricate the drilling tool. Consequently, there will be a drilling set with two passages in the case of a single-fluid system, a set with three passages in the case of a double-fluid system, and so on. The alternative approach is to form a preliminary hole into which the jet grouting set with a monitor can be inserted subsequently, without the possibility of lubricating a drilling tool. See, for example, JP 10 195862.
In many conventional monitors, the cement mixture required for the jet treatment tends to block the channels used for the supply of lubricating fluid to the drilling tool. When the mixture has hardened, the low pressure at which the lubricating fluid is injected is insufficient to clear the channels leading to the tool; consequently, the monitor must be dismantled for each new drilling cycle in order to remove the residues of hardened cement mixture from it. Independent pumps are also required to provide the supply through the drilling pipe and the jet grouting pipe.